Bidirectional AC-DC Converter Research Articles

Development and Application of a Fuzzy Control System for a Lead-Acid Battery Bank Connected to a DC Microgrid

This study presents the development and application of a fuzzy control system (FCS) for the control of the charge and discharge process for a bank of batteries connected to a DC microgrid (DC-MG). The DC-MG runs on a maximum power of 1 kW with a 190 V DC bus using two photovoltaic systems of 0.6 kW each, a 1 kW bidirectional DC-AC converter to interconnect the DC-MG with the grid, a bank of 115 Ah to 120 V lead-acid batteries, and a general management system used to define the operating status of the FCS. This FCS uses a multiplexed fuzzy controller, normalizing the controller’s inputs and outputs in each operating status. The design of the fuzzy controller is based on a Mamdani inference system with AND-type fuzzy rules. The input and output variables have two trapezoidal membership functions and three triangular membership functions. LabVIEW and the NI myRIO-1900 embedded design device were used to implement the FCS. Results show the stability of the DC bus of the microgrid when the bank of batteries is in the charging and discharging process, with the bus stabilized in a range of 190 V ± 5%, thus demonstrating short response times to perturbations considering the microgrid’s response dynamics.

Soft-Switched Modulation Techniques for an Isolated Bidirectional DC–AC

Two carrier-based unipolar-sinusoidal pulse width modulation (SPWM)-oriented modulation techniques for an isolated bidirectional dc–ac converter are proposed, compared, and validated in this paper. The dc–ac converter is composed of a full-bridge (FB) inverter cascaded with a cycloconverter through a high-frequency transformer. Both modulation techniques proposed in this paper can realize zero-voltage switching (ZVS) for the FB inverter and zero-current switching or ZVS for the cycloconverter in all load range, and are able to suppress the voltage spikes introduced by the transformer leakage inductance as well. In order to increase the converter efficiency and power density, we propose to utilize SiC MOSFETs for the converter. The first modulation technique enables the utilization of Si-SiC hybrid switches with no synchronous rectification (SR), for the purpose of lowering the converter cost. The second modulation technique requires all switches to be SiC MOSFETs, but with SR, which increases the converter efficiency. A 400-V dc to 240-V ac 1.2-kW prototype has been developed to validate the effectiveness and performance of the proposed carrier-based unipolar-SPWM-oriented modulation techniques.

Analysis and Implementation of Closed-Loop Control of Electrolytic Capacitor-Less Six-Pulse DC-Link Bidirectional Three-Phase Grid-Tied Inverter

Cascaded bidirectional dc–ac converters are commonly used in uninterruptible power supply applications and battery chargers for electric vehicles. Power conversion units for such applications employ a large electrolytic capacitor at high-voltage dc bus, which not only reduces the lifetime but also adds to the weight of the converter. In this paper, a novel bidirectional dual active bridge cascaded three-phase converter (DABCC) with six-pulse dc link is proposed. Also, a new closed-loop control scheme to implement the six-pulse modulation (SPM) technique in DABCC is proposed. The advantages of the proposed control scheme in DABCC are that the electrolytic capacitor is eliminated and is replaced with a low-value film capacitor resulting in increased reliability, compactness, and reduced cost. SPM technique also increases the dc bus utilization and decreases the inverter average switching frequency to 33% when compared with the conventional sine pulse-width modulation (SPWM). Harmonic components introduced in the pole voltages of the SPM-modulated DABCC due to inverter dead time are analyzed, and a resonant controller is designed to mitigate them. Also, root mean square current stress for high-voltage dc-link capacitor is studied, and it is shown that the capacitor requirement for SPM modulation is lower than the conventional SPWM. To validate the proposed control scheme, an 800-W proof-of-concept laboratory hardware prototype is fabricated and experimental results are demonstrated.

Improved Control Strategy for Bidirectional Single Phase AC-DC Converter in Hybrid AC/DC Microgrid

This paper presents the improved control strategy with optimal switching scheme to improve the bidirectional converter's power flow control capability and to allow for different modes of operation in hybrid AC/DC microgrid. Also, this improved control strategy eliminates the need of PI controller for independent reactive power control. With the improvements in digital controllers, it is possible to implement complex control circuitry easily. The performance of bidirectional converter with improved control strategy is verified first in simulation environment and then on lab scale experimental set-up with the control circuit is implemented using hardware in loop in the real-time digital simulation platform. The simulation and experimental results for all the cases show that the controller is highly capable for power transfer between AC and DC grid with very fast response and also control reactive power without additional PI controller. Also, it maintains the current total harmonic distortion (THD) within standard limits.

Soft-Switching PWM Technique for Grid-Tie Isolated Bidirectional DC–AC Converter With SiC Device

Soft-switching techniques have become very attractive in bidirectional grid-tie dc–ac converters utilizing high-frequency link transformers for galvanic isolation. This paper presents a new pulse-width modulation (PWM) switching technique for controlling a bidirectional isolated dc–ac–ac converter along with its soft-switching method. The proposed PWM technique has the ability to control the input dc current and to inject a sinusoidal three-phase current to the grid at unity power factor. In the first stage, an H-bridge converter is used to convert the dc voltage to a high-frequency square-wave single-phase voltage. In the second stage, a matrix converter is used to convert the high-frequency voltage waveform to conventional three-phase voltage synchronized with the grid. Therefore, a single-phase high-frequency transformer is used to link the H-bridge output voltage to the matrix converter input voltage. The proposed soft-switching technique is achieved by connecting shunt ceramic capacitors across all converter switches. The mathematical model and the circuit operation for soft-switching are presented along with the voltage controllable limits. The effectiveness of the proposed technique has been verified experimentally using a laboratory prototype.

Power control in centralized distributed AC load for wind energy system

In electrical power systems, isolated energy system is a more challenging task to fulfill the active power demand as well as reactive power demand under the fluctuating wind at different load conditions. In this paper, a centralized AC load, centralized distributed hybrid system (CDHS), is analyzed. CDHS has the following features in place of the DC centralized grid system, which is focused on controlling of a permanent magnet synchronous generator, a bi-directional DC-AC converter (BDC) with battery energy storage, and a wind energy conversion system with a single-stage, 3-phase 4-wire AC-AC converter connected to a common AC bus. Non-isolation of the input terminals, multi-stage operation, size and cost of the circuits, and efficient working of the AC-AC converter has been resolved using a 3-phase 4-wire AC-AC converter. A specific triggering pattern of variable frequencies to a constant frequency has been generated for the switching devices in the AC-AC converter to achieve a desired polarity of the output voltage. A BDC connects the isolated load using a natural frame instantaneous current control technique to manage the active and reactive load power flow among the parallel connection AC hybrid systems. Different conditions such as a constant load and a variable load for variable source supplies are demonstrated for the architecture of CDHS in MATLAB/Simulink. The modelling of CDHS and performance analysis have been disseminated out that the proposed CDHS is capable of managing the active power demand with reactive power for voltage variations.

A Droop Control Based Three Phase Bidirectional AC-DC Converter for More Electric Aircraft Applications

This paper addresses the design and implementation of a droop controlled three phase bidirectional AC-DC converter for more electric aircraft (MEA) applications. A unified control strategy using a droop characteristic is presented to achieve the bidirectional power flow between the AC and DC source while maintaining a stable DC output voltage. Based on the rigorous control design, the converter is capable of operating over a wide frequency range with a unity power factor. The key findings obtained from the theoretical analysis are confirmed by simulation studies and further validated by a scaled down laboratory prototype. The practical results show good performance of the bidirectional converter and demonstrate the effectiveness of the proposed control strategies.

Low-Frequency Common-Mode Voltage Control for Systems Interconnected With Power Converters

In this paper, an active common-mode (CM) duty cycle injection method is proposed to control the dc and low-frequency CM voltage in grounded systems interconnected by power converters. Specifically, a 380-V dc nanogrid and a single-phase ac distribution system interfaced with a transformerless ac–dc converter is considered. In such architecture, the ac and dc CM quantities are coupled through the ground; while the high-frequency noise is filtered by passive components, the dc and low-frequency CM voltage need be controlled to generate symmetric dc-bus voltages and effectively mitigate the ground leakage current. Using a two-stage bidirectional ac–dc converter as an example, the operating range of the proposed method is identified under different ac and dc voltages and alternative grounding schemes. Further, a CM equivalent circuit is derived and experimentally validated, and used to design the controller of the proposed control system. Experiments with resistive and constant power loads in rectification and regeneration modes are used to validate the performance and stability of the control method. As a result, the dc-bus voltages are rendered symmetric with respect to ground, and the leakage current is suppressed. Finally, the control method is generalized to three-phase ac–dc converters for larger power systems.

Design and Optimization of an Efficient (96.1%) and Compact (2 kW/dm3) Bidirectional Isolated Single-Phase Dual Active Bridge AC-DC Converter

The growing attention on plug-in electric vehicles, and the associated high-performance demands, have initiated a development trend towards highly efficient and compact on-board battery chargers. These isolated ac-dc converters are most commonly realized using two conversion stages, combining a non-isolated power factor correction (PFC) rectifier with an isolated dc-dc converter.This, however, involves two loss stages and a relatively high component count, limiting the achievable efficiency and power density and resulting in high costs. In this paper, a single-stage converter approach is analyzed to realize a single-phase ac-dc converter, combining all functionalities into one conversion stage and thus enabling a cost-effective efficiency and power density increase. The converter topology consists of a quasi-lossless synchronous rectifier followed by an isolated dual active bridge (DAB) dc-dc converter, putting a small filter capacitor in between. To show the performance potential of this bidirectional, isolated ac-dc converter, a comprehensive design procedure and multi-objective optimization with respect to efficiency and power density is presented, using detailed loss and volume models. The models and procedures are verified by a 3.7kW hardware demonstrator, interfacing a 400Vdc-bus with the single-phase 230V,50Hz utility grid. Measurement results indicate a state-of-the-art efficiency of 96.1% and power density of 2 kW/dm3, confirming the competitiveness of the investigated single-stage DAB ac-dc converter.

Bidirectional Single Power-Conversion DC–AC Converter With Noncomplementary Active-Clamp Circuits

This paper presents a bidirectional single power-conversion dc–ac converter with noncomplementary active-clamp circuits. The proposed converter comprises a bidirectional flyback converter and an unfolding bridge. In order to interface the grid with a low-voltage energy storage through only single power-conversion, the bidirectional flyback converter transforms the low voltage directly into the folded grid voltage and regulates the folded grid current. The proposed converter adopts noncomplementary operation strategy for the active-clamp circuits. By using this strategy, the bidirectional flyback converter not only avoids the voltage spike but also minimizes the power losses by the circulating energy. Thus, with single power-conversion and noncomplementary active-clamp circuits, the proposed converter obtains high power efficiency. To facilitate the bidirectional single power-conversion, a novel control algorithm is developed. With this control algorithm, the proposed converter ensures high grid power quality and seamless mode transition. The proposed bidirectional dc–ac converter is theoretically analyzed in detail. The experimental results based on a 250-W prototype are provided to evaluate its performance.

Control and Modulation of Bidirectional Single-Phase AC–DC Three-Phase-Leg SPWM Converters With Active Power Decoupling and Minimal Storage Capacitance

In a single-phase grid-connected nanogrid system, a bidirectional ac–dc converter is usually required to transfer energy between the ac grid and a dc bus. Active ac power balancing is often implemented to prevent the ac grid ripple power from injecting into the dc bus. A four-phase-leg sinusoidal pulsewidth-modulation (SPWM) converter can readily provide power factor correction and active ac power balancing. In this paper, the use of three-phase-leg SPWM converters for achieving these functions is analyzed. A family of bidirectional single-phase ac–dc three-phase-leg SPWM converters with an ac storage capacitor for use in a nanogrid system is designed with a general control structure and a modulation scheme for minimizing the ac storage capacitance. The general control structure is designed to achieve a decoupled system of a power-factor-correction converter cascaded with an active ac power load at the dc bus. The decoupled system is developed based on decomposition to differential-mode and common-mode voltages. The modulation method involves an extra zero-sequence voltage injection derived from the three-phase-leg SPWM voltages without introducing higher order harmonic distortions. A significant reduction of the ac storage capacitance and an improvement of converter efficiency are achieved. The design and analysis are verified by simulations and experimental measurements.

Robust Regenerative Charging Control Based on T–S Fuzzy Sliding-Mode Approach for Advanced Electric Vehicle

This work presents a robust regenerative charging (RC) control scheme for the brushless dc (BLDC) motor drives in advanced electric vehicle (EV). After analyzing the equivalent circuit of the bidirectional ac–dc converter, we first derive the mathematical model under RC mode by using state-space averaging method. Then, we originally formulate the Tagaki–Sugeno (T–S) fuzzy model to represent its nonlinear dynamics. By combining the merits of T–S fuzzy technique with sliding-mode control (SMC) method, we develop a T–S fuzzy SMC (TSFSMC)-based constant-voltage (CV) charging control to guarantee both high performance and robust stability. Moreover, comparing to conventional methods, we declare that TSFSMC does not need the input channel to be identical and can simultaneously achieve dual goals of electric braking and RC without any additional devices. We have implemented a control prototype to analyze and confirm the validity of TSFSMC. Results show that high charging performance and significant efficiency improvement are obtained.

Modified Model Predictive Control of a Bidirectional AC–DC Converter Based on Lyapunov Function for Energy Storage Systems

Energy storage systems have been widely applied in power distribution sectors as well as in renewable energy sources to ensure uninterruptible power supply. This paper proposes a modified model predictive control (MMPC) method based on the Lyapunov function to improve the performance of a bidirectional ac–dc converter, which is used in an energy storage system for bidirectional power transfer between the three-phase ac voltage supply and energy storage devices. The proposed control technique utilizes the discrete behavior of the converter, considering the unavoidable quantization errors between the controller and the control actions selected from the finite control set of the bidirectional ac–dc converter. The proposed control method reduces the execution time delay by 18% compared with the conventional model predictive control. Moreover, the nonlinear system stability of the proposed MMPC technique is ensured by the direct Lyapunov method and a nonlinear experimental system model. Detailed experimental results with a 2.5-kW downscaled hardware prototype are provided to show the efficacy of the proposed control system.

A Three-Phase Bidirectional ACDC Converter with Y-Δ Connected Transformers

This paper proposes a three-phase bidirectional ac/dc converter with Y–Δ connected transformers. The converter achieves both buck-boost ac/dc bidirectional conversion and electrical isolation with the single-stage structure. By introducing three dc–dc transformers, the total power is distributed and the current stress of the rectifier stage decreases. The Y–Δ connected transformers provide extra step-down conversion ratio and facilitate the converters’ application in buck conversion. The circuit derivation, operation principles, and control strategy with the SVPWM algorithm are presented. A 3-kW prototype with 380-V dc and a 1.6-kW prototype with 48-V dc were built in the lab and experimental results verify the theoretical analysis well.

High-Efficiency Isolated Bidirectional AC–DCConverter for a DC Distribution System

High-Efficiency Isolated Bidirectional AC–DCConverter for a DC Distribution System

Reliable transformerless battery energy storage systems based on cascade dual‐boost/buck converters

In this study, the cascade dual-boost/buck half-bridge and full-bridge bidirectional ac–dc converters are proposed for grid-tie transformerless battery energy storage systems (BESSs). The proposed converter contains the advantages of the traditional cascade H-bridge (CHB) converter. However, compared with CHB converter, there is no shoot-through issue in the proposed converter, which is more reliable for BESS. In addition, a unified control scheme is proposed for active-power control and individual state-of-charge-balancing control of BESS, which is very modular and easy to implement. At the end, a laboratory BESS is constructed and tested combining a cascade two-unit dual-boost/buck half-bridge converter with nine lead–acid series-connected battery units. Experimental results obtained from the laboratory system verified the feasibility and effectiveness of the proposed BESS and unified control scheme.

Effective Test Bed of 380-V DC Distribution System Using Isolated Power Converters

This paper proposes an effective test bed for a 380-V dc distribution system using isolated power converters. The proposed test-bed system is composed of a grid-interactive ac–dc converter for regulating the dc-bus voltage, a bidirectional converter for the battery power interface, a renewable energy simulator, dc home appliances modified from conventional ac components, a dc distribution panel board, and its monitoring system. This paper discusses three isolated power converters, i.e., a bidirectional ac–dc converter, a bidirectional dc–dc converter, and a unidirectional dc–dc converter for the effective power interface of a dc bus. These isolated power converters are designed using a dual-active-bridge converter and the resonant topologies of $CLLC$ and $LLC$ . The proposed test-bed system was implemented using a 5-kW bidirectional ac–dc prototype converter, a 3-kW bidirectional dc–dc prototype converter, and a 3-kW unidirectional dc–dc prototype converter. Finally, the performance of the test-bed system has been verified using practical experiments of load variations and bidirectional power flow, employing the prototype converters.

LMI approach of state-feedback controller design for a STATCOM-supercapacitors energy storage system associated with a wind generation

The influence of a constant speed wind turbine in an electrical network during torque impact can lead to various problems. The disturbances affect active power quality, reactive power and r.m.s. phase to phase voltage. In this paper, we propose an original system and control to solve firstly the problems due to the variation of DC-voltage source and secondly the brutal impact of active and reactive powers. The principle of the proposed wind energy system with STATCOM supercapacitor energy storage is to limit the rapid variation of active power and give the required reactive power compensation to the induction machine during the torque change. The supercapacitor is directly coupled to the common DC-link of the whole bidirectional DC–AC converter. The control strategy of the STATCOM uses a Linear Matrix Inequalities (LMIs) approach to regulate the currents injected or absorbed by the inverter. The same strategy is used to control the variable DC supercapacitor voltage. The simulation results of the STATCOM using a state-feedback with integral controller for id,iq currents of the converter and a simple proportional (Kp) controller cascaded to d axis current loop for DC voltage source are presented and discussed. Finally, implementation in real time is performed and the first experimental results are presented to validate the proposed control.

Model Predictive Control of Bidirectional AC-DC Converter for Energy Storage System

Energy storage system has been widely applied in power distribution sectors as well as in renewable energy sources to ensure uninterruptible power supply. This paper presents a model predictive algorithm to control a bidirectional AC-DC converter, which is used in an energy storage system for power transferring between the three-phase AC voltage supply and energy storage devices. This model predictive control (MPC) algorithm utilizes the discrete behavior of the converter and predicts the future variables of the system by defining cost functions for all possible switching states. Subsequently, the switching state that corresponds to the minimum cost function is selected for the next sampling period for firing the switches of the AC-DC converter. The proposed model predictive control scheme of the AC-DC converter allows bidirectional power flow with instantaneous mode change capability and fast dynamic response. The performance of the MPC controlled bidirectional AC-DC converter is simulated with MATLAB/Simulink® and further verified with 3.0kW experimental prototypes. Both the simulation and experimental results show that, the AC-DC converter is operated with unity power factor, acceptable THD (3.3% during rectifier mode and 3.5% during inverter mode) level of AC current and very low DC voltage ripple. Moreover, an efficiency comparison is performed between the proposed MPC and conventional VOC-based PWM controller of the bidirectional AC-DC converter which ensures the effectiveness of MPC controller.

Cell Equalization System for Li-Ion Battery Management Based on Fly-Back DC/AC Convertor

Cell equalization acts an important role on the battery life preserving for space application. This paper describes a lithium ion battery cell equalization method for spacecraft that employs innovative design features in power subsystem. The scheme proposed for balancing the battery pack relies on energy conversion devices as it uses transformers to move energy from high voltage cells to low voltage cells without energy loss. The method employs bi-directional forward DC-AC converters, plus a unique common node to provide autonomous charge distribution. It also supports individual cell voltage monitoring, telemetry data. Multisim is used to analyze the function and performance, experimental result proves that the method is effective for Lithium Ion battery equalization.